Underwater ammunition

ABSTRACT

Underwater ammunition is disclosed, in which a flechette projectile is enclosed within the air-filled bore of a watertight sealed disposable barrel adapted to be removably inserted into a carrier cylinder or block for firing. The projectile is suspended by and between a rear-end-cradling pusher sabot, and a frangible guiding-and-supporting unit including a soft rubber nosesupporting cradle slug having a forward relieved surface. A propellant charge and percussion primer are disposed rearwardly of the pusher sabot which is initially anchored by a shear pin, and the projectile is propelled forward by primer ignition of the propellant to shear the shear pin and impart forward motion to the pusher sabot, which in turn drives the projectile through a frangible sealed front barrel convexly domed cover. Forward motion of the sabot is stopped by successive incremental energy absorption through the shearing of an external helical shear thread on the forward section of the pusher sabot at the forward end of its travel, the shearing and compaction of which shear thread, as well as the resultant compaction of a soft malleable length of material in the thread grooves, also seals in the propellant gases within the barrel bore at the end of the sabot forward travel. One or more pressure dropping annular expansion grooves on the rear section of the pusher sabot are employed to obviate blowby of propellant gases during forward motion of the sabot.

United States Patent [111 3,585,934

[72] Inventors Kenneth E. Mueller Primary Examiner-Verliri R. Pendegrass Baltimore; Attorney-Reginald F. Pippin, Jr.

John L. Criteher, Cockeysville, both ol,

Md. [2!] Appl. No. 688,646 ABSTRACT: Underwater ammunition is disclosed, in which a [22] Filed Nov. 24, 1967 flechette projectile is enclosed within the air-filled bore of a [45] Patented June 22, 1971 watertight sealed disposable barrel adapted to be removably [73] Assignee Ml Corporation inserted into a carrier cylinder or block for firing. The projec- Cockeysville, Md. tile is suspended by and between a rear-end-cradling pusher sabot, and a frangible guiding-and-supporting unit including a soh rubber nose-supporting cradle slug having a forward re- [54] UNDERWATER mmo lieved surface. A propellant charge and percussion primer are 14 ClalnuJDrawlng Fig. disposed rearwardly of the pusher sabot which is initially anchored by a shear pin, and the projectile is propelled for- [52] U.S.Cl. 102/48, ward by Primer ignition of the propellant to Shem, the Shear 102/43 42/1 pin and impart forward motion to the pusher sabot, which in a a Fleldo Search rel convexly domed cover. Forward motion of the sabot is 95; 89,192; 42/] stopped by successive incremental energy absorption through the shearing of an external helical shear thread on the forward [56] References CM section of the pusher sabot at the forward end of its travel, the

UNITED STATES PATENTS shearing and compaction of which shear thread, as well as the Brandt resultant compaction of a sofi malleable length of material in 2,336,819 12/1943 Turechek 89/ l (.02) h h d ooves, also seals in the propellant gases within the 3 1 960 P0116! 102/38 X barrel bore at the end of the sabot forward travel. One or more 9 1 9/1 Anspacher l02/93 pressure dropping annular expansion grooves on the rear sec- 3, 1 Taylor tion of the pusher sabot are employed to obviate of Bclcher et al propellant gases during forward motion of the sabot n l5d w Z:

.2 WM A b I50 37a 37b 39 390 Be UNDERWATER AMMUNITION Briefly, according to this invention underwater ammunition is provided in which a projectile is contained in a watertightsealed air-filled cartridge forming its own barrel, the barrel being closed at the forward end by a domed frangible cover seal, and the propellant blast being sealed in by a retained pusher sabot to prevent eardrum damage to an underwater operator. The projectile is mounted in the barrel-forming airfilled cartridge for .efilcient power utilization on firing and thereby yielding maximum initial muzzle velocity. The pusher sabot is initially anchored by a shear pin to aid in uniformity of interior ballistic performance, and at the forward end of the barrel the nose is cradled in a sofi elastic slug relieved on its forward surface and having a thin membrane between the rear face and forward relieved surface of the slug.

Details and various other objects, features and attendant advantages of the invention will become apparent to those skilled in the art from a reading of the following detailed description of a single preferred embodiment constructed in accordance with the invention, taken in conjunction with the accompanying drawing wherein:

The FIGURE is a longitudinal section view of a round of ammunition according to the invention.

Referring now in detail to the drawing, the illustrated round of underwater ammunition generally indicated at 11 includes a long thin subcaliber, drag or tin-stabilized flechette projectile enclosed within the air-filled bore 131: of a tubular cartridge which forms its own disposable barrel 13 and which is watertight sealed at its opposite ends.

The projectile 15 is suspended by and between a dual purpose pusher piston sabot 17 of metal, such as steel, or other suitable material, a frangible guiding-and-supporting unit 31, 33, 35, and a nose cradle 37 of soft elastic rubber.

The tubular cartridge case barrel 13 has a brass frangible front closure cap seal 41 and a threadedly secured head or end cap 19 containing a primer 21, a primer anvil restrainer plug 22, and a propellant charge 23.

The round is waterproofed for hydrostatic pressures in substantial excess of the desired useful depth range of for instance up to 30 feet through several structural measures, including application of a waterproof compound such as silastic of the room temperature vulcanizing type (an example of which is marketed under the name RTV), to the various exposed forward and rear joint lines. Also, to this end, head end cap 19 has along at its forward reduced diameter section a threaded surface 19a which engages a complementary internal threaded surface in the barrel 13, which threaded surfaces provide for ease of assembly and securing of the headed end cap 19 with the barrel 13. Threaded surface 19a and the abutting forward surface of head flange 19b are sealed to the cartridge barrel 13 with a-silicone adhesive, or other suitable waterproof sealant, such as resin-type vsealants, to provide watertight sealing of this joint.

End cap 19 houses at its forward end a charge of propellant 23 and at its rearward end a percussion primer 21. Percussion primer 21 is secured in place by a press fit primer restrainer plug 22 having a central flash hole 220 communicating between the primer and the propellant charge 23, and by an annular retaining lip 19f formed in the end cap and which serves as a shoulder against which the primer is seated to prevent rearward blowout of the primer upon firing. The propellant charge is preferably suitably enclosed at its forward end by a thin frangible propellant cover cap 19c, of frangible plastic or other suitable material, paper or rubber, press fit into the forward end of the head end cap 19, in order to provide for ease of subassembly and assembly, and is also particularly useful in preventing particles of the propellant charge from leaking into clearance spaces between the metal parts in the course of handling operations, and thereby avoiding possible accidental ignition of the propellant charge.

One or more spaced temperature-reducing gas expansion grooves 17a are formed at the rear end of the pusher piston sabot l7 and serve, in conjunction with a light piston-to-bore lateral clearance of for instance 0.00l0.002 inch, to provide a form of air-bearing friction-reducing sliding fit for the pusher piston 17 as well as enabling the adiabatic expansion and temperature reduction of the propellant gases during the movement of the piston and projectile forward by the pressure of these gases. The grooves may be formed of a suitable depth to insure that propellant gases will be sufficiently restricted in passage and reduced in pressure that substantially no propellant gas will be enabled to escape therepast during the time of travel of the pusher piston sabot 17 from its initial position to its final terminal position. For instance, with a standard propellant charge sufficient to propel a -grain projectile forward at a muzzle velocity of 700 feet per second it has been found that one pressure and temperature reduction groove 17a of 0.070 inch width and 0.070 inch depth is generally adequate in a piston of 0.374 inch outer diameter, with a stroke length of approximately 4 inches for the piston 17.

The forward section of the pusher sabot 17, which is formed of metal such as steel, or other suitable material, has an impact energy absorbing shallow external helical square shear thread 17c formed thereon with a plurality of turns, and forward of the threaded portion is a reduced diameter end section 17f which is chamfered as at 173. At the terminal zone of travel of the pusher sabot in the barrel bore 13b the helical energy absorbing thread turns are incrementally sheared by sequen tial incremental contact with an axially relatively thick but radially narrow inwardly extending annular shoulder stop 13c formed at the end of the barrel. Thus, the impacting of the threaded turns against the shoulder stop 13c effects a desired energy absorption and stopping of the pusher sabot at the forward end of the barrel bore 13b, the upsetting and compacting of the sheared thread material serving to effect a further jam seal of the propellant gas against undesired escape into the water after completion of the firing action and expulsion of the projectile from the barrel. While in most instances the plural threaded turns 17c are in themselves sufficient to provide full energy absorption for effective stopping of the pusher sabot within the barrel 13, it is desirable that in addition one or more additional spaced energy absorbing continuous rings 17!; be provided rearward of the threaded turns 170 to provide for any excess energy absorption which might be required due to variation in propellant charge or strength of materials. These excess energy-absorbing spaced circumferential rings 17b may be of one or two in number, as illustrated, and provide abrupt intermittent impact energy absorption as distinguished from the smooth continuous energy absorption provided by the forward thread turns 17c, and due to their relatively much large resistive action area they require greater energy for shearing than is required at the incremental thread contact zones, thus effectively providing a full and sure stop medium for any small excess energy which may be involved.

Sealing of the spent propellant gases is further effected by disposing in one or several turns of the helical grooves between the helical thread 170 a length of soft wire 18 such as soft copper or similar malleable material press-fitted into location, the illustrated embodiment having approximately three successive turns, and with the outside diameter sized to match that of threads 17c. Improved sealing is thus obtained when the shearing deformation of threads 17c occurs, as the stacking tendency of threads 17c during energy absorption forcibly forms or extrudes the soft copper wire into microscopic voids, thereby further aiding in the sealing of the residual propellant gases inside the cartridge barrel.

The sabot 17 has a hollow open-ended forward facing cavity, the rear portion of which forms a pocket 17d which serves to cradle and support the rear tail section 15b, 15a, of the projectile 15, the pocket fitting about the shroud 15c and fins 15b in a close slip fit, as for instance of the order of 0.003 inch clearance. In addition, the enlarged open-ended cavity formed in forward end of the piston sabot 17 serves to reduce weight, thereby reducing the propellant charge requirement for propulsion and the energy absorption requirement for sabot travel termination.

Pusher sabot 17 is initially anchored to the head end cap 19 by shear pin 20 prior to firing of the cartridge, the anchor attachment being effected as by disposition of the shear pin diametrally through diametral bore openings in a rear facing annular lip extension 17k formed on the sabot 17, with the opposite anchor ends of the shear pins extending into complementary diametral openings 19!: formed in the forward section 19p of head end cap 19. This shear pin anchor connection serves the dual function of preventing dislodgement of the sabot in the event of dropping of the cartridge, and also aids in effecting more uniform internal ballistics in that a predetermined pressure must be attained in the propellant firing chamber before the sabot can be moved forward, thereby ensuring substantially full propellant burning, higher muzzle velocity, and more uniform muzzle velocity of the projectile 15.

The frangible dual purpose guiding-and-supporting unit 31, 33, 35, for the projectile serves to provide nose and midsection guiding of the projectile during pushing thereof by the sabot 17 while the projectile travels along the bore 13b, and to this end this frangible unit takes the form of a thin metal centering disc 33 formed of heat-treated aluminum for radial structural rigidity, which may suitably be of the order of 0.0l0.0l2 inch thick. The centering disc 33 serves to resist and carry radial side loads from the projectile shank d, and has a central hole formed therein which forms a slip fit with the cylindrical projectile shank 15d and may be for instance of the order of 0.001 inch oversize for this purpose. Disposed in lateral supporting relation against the opposite end faces of the frangible centering disc 33 are two lightweight frangible supporting discs formed of lightweight foam plastic, such as styrofoam, and which serve to provide limited lateral stability to the centering disc 33. Each of the disc 31, 33, and 35 are slidably complementary to the barrel bore 13b, the thin centering disc 33 being of the order of 0.001 inch undersize and the thicker lateral supporting discs 31, 35 having a radial clearance of the order of approximately 0.0l0 inch to insure adequate centering action with ease of insertion into and ultimate expulsion from the bore 13b. The lateral supporting styrofoarn discs 31, 35 may be removably secured about the shank 15d of the projectile as by press fitting thereon, or by use of adhesive as may be desired.

The aluminum centering disc 33 is relatively thin and easily frangible; however, it is desirable to increase the frangibility of this disc while maintaining its radial centering strength to a substantial degree, as by radial scoring of one or both lateral surfaces of the disc 33. This is particularly useful in enabling ease of expulsion of the centering disc from the bore 13b, 13d of the barrel 13 and separation from the projectile 15.

The tapered blunt nose end 150, 15a of the projectile 15 is cradled against shock during handling, such as upon dropping of the ammunition, particularly when impact occurs at the forward end of the barrel 13, through the medium of a slug 37 of soft rubber or other suitable soft elastic material. Slug 37 has athin rear membrane 37d, e.g. 0.0l00.030 inch for the size projectile discussed above, which receives and cradles the blunt nose end 150, 15a of the projectile. in the cradled prefired position of the projectile within barrel 13 the membrane 37a and surrounding surface of the slug are depressed under compression by the blunt nose end 15a, 15a of the projectile, thereby forming a compression cradle recess 37a for the nose of the projectile. The slug is relieved on its forward surface through formation of an outwardly flared frustoconical recess 37c. This flared relief recess 370 in conjunction with a bore 37b communicating between membrane 37d and forward relief recess 37c enables the projectile to pass easily through the slug 37 when fired without altering the accuracy of the projectile flight. The short bore section 37b is of smaller diameter than the flat end face 15a of the projectile nose 15a, the thin membrane 37d and this short center section of the slug 15 in conjunction with relief recess 37c serving the dual function of elastic forward partial deflection and elastic retention of the projectile nose when the cartridge 11 is dropped on its forward end, while enabling puncture and effective free passage of the projectile therethrough when propelled forward upon firing of the cartridge.

A thin frangible retention washer 39 having a central opening 39a and formed as of generally rigid plastic such as melamine resin, or other suitable material, is disposed between the soft elastic cradle slug 37 and retention shoulder stop formed in the barrel. This retention washer prevents dislodgement of the soft elastic cradle slug 37 in its impact retaining action, as the shoulder 130 is relatively narrow and the soft elastic slug could otherwise be deflected free of the shoulder 13c in incautious handling of the ammunition.

The forward end of the barrel 13 is closed by a domed closure cap 41 of metal, such as brass, which is secured in watertight relation within the annular shoulder bore end surface 13d, as by an annular solder joint as indicated at 41a. As will be noted from the FIGURE, the nose end 15a of the projectile 15 is spaced from this closure cup in the unfired ammunition assembly. The domed shape of closure cap 41 imparts the necessary structural rigidity and axial deflection resistance to prevent inward crushing of the dome by impulse loads resulting from firing of adjacent rounds underwater.

The ammunition may be used in lightweight cylinders or other carrier elements, with the body of the cartridge forming its own pressure absorbing and retention barrel 13. Thus, in operation, the cartridge assembly 11 may be removably inserted loosely into a bore chamber, and the cartridge 11 may be suitably removably retained in the cylinder bore through the medium of a conventional breech and a retaining lip at the forward end of the cylinder bore chamber, the percussion primer being fired preferably through the medium of a spherical firing pin to prevent rupture of the primer and escape of propellant gases. The flash from the primer flashes through the flash hole 22a and ignites the propellant charge 23, which in turn burns and forms expanding propellant gas, the pressure from which is confined by the walls of the barrel l3 and the attached head end cap assembly 19. Upon the propellant gas reaching a predetermined pressure the shear pin is sheared and the pusher piston sabot 17 is moved forward to propel the projectile 15 toward and through the closure cup 41. The pusher sabot prevents gas leakage therearound during its passage along the barrel bore 13b, through the medium of the labyrinth seal pressure and temperature reducing groove or grooves 17a. The hot gases leaking around the sabot rear surface into the groove(s) 17a are retarded through sequential adiabatic expansion into the groove volumes, thereby reducing by adiabatic expansion the gas temperature and pressure, and locally resulting in a delay of forward gas motion therepast that is longer than the power stroke period of the round. In addition, the labyrinth groove arrangement and associated propellant gas flow thereabout enables the sabot 17 to utilize a floating air-bearing type clearance of for instance 0.00l0.002 inch with the barrel bore 13b for friction reduction and increase in power utilization efficiency.

During the forward motion of the sabot 17 and projectile 15, the projectile pierces the elastic slug 37 and closure cap 41. The foam plastic and aluminum centering disc unit 31-35 maintains the projectile 15 centered long enough during its acceleration and passage through the bore 13b to establish an accurate launch, as once the projectile has attained a high velocity within the cartridge its inertial stability no longer requires alignment by the disc, the latter being the situation at the terminal zone of travel of the projectile through and from the barrel bore.

The extremely light construction and ease of frangibility of the centering discs 31-35, as well as the soft elastic nature and relieved construction of the retention slug 37, are advantageous in their not being effective to materially perturbate the alignment of the rapidly moving projectile as the slug 37 is punctured and deflected, and the discs 31-35 and slug 37 are ultimately collapsed and expelled through the reduced diameter end bore 13d of the barrel 13.

The pusher sabot-projectile assembly l7, l5 continues to the end of the cartridge bore 13b, where the shoulder stop 13c is impacted by the shallow square thread 17c on the sabot 17, which thread matches the thickness of the shoulder stop 130. The cartridge barrel shoulder stop He being much thicker than a single mm of the thread 17c, the impact of the shear thread 170 thus effects a continuous shearing of the sequential incremental sections of the thread. As each increment of thread 17c shears, it stacks against the shoulder stop 131:, while the sabot 17 continues forward through the bore 13b, 13d, and thus as each turn is incrementally sheared it stacks on itself and incrementally serves as a stop for shearing of the next turn of thread, smoothly and at a substantially constant energy absorbing rate due to the helical arrangement and continuous incremental sequential shearing action. This results in a maximum of damping energy with a minimum continuing restraining force being required by the cartridge barrel during deceleration to prevent the sabot from exiting from the cartridge, and desirably a surplus quantity of thread 170 exists to insure stopping for a given propellant charge. However, as previously noted, in the event of excess energy imparted to the sabot 17 by the propellant 23, the intermittent impacted excess energy absorbing separated rings 17b will be effective to fully absorb the excess energy and effect stoppage of the sabot travel and retention of the sabot within the barrel 13, thereby also fully sealing and trapping the residual gases which may remain. The prevention of escape of propellant gases during and after termination of the travel of the sabot is important in preventing eardrum damage to the operator under water.

In the initial travel of the sabot 17 and projectile along the length of the bore 13b, the travel of the sabot is unimpeded by any water mass, as the water is sealed out of the bore 13b by the closure cap 41 prior to firing. When the sabot begins to slow down upon impacting the shoulder 13c the projectile velocity is thus at its highest value, whereby momentum of the projectile 15 thus causes the projectile to separate from the sabot and continue on out of the cartridge barrel 1?: and along its course of trajectory through the water.

The frangible guiding and centering discs 31-35 are shattered by impacting thereof by the forward end of the sabot 17. The deflected slug 37 and the residual components or fragments of the discs 31-35 and washer 39 and the closure cap 41 expel from the bore 13b, 13d with the projectile, and the drag force of the water thereon easily strips these residual fragments free of the projectile such that the flight alignment of the projectile is substantially for all practical purposes undisturbed.

That which we claim is:

l. Underwaterammunition comprising:

a disposable barrel having a longitudinal bore closed at both ends and having a primer and propellant disposed therein at one end and a frangible closure at its opposite end,

a blunt tapered nosed projectile disposed in an air chamber formed in said bore between said propellant and said frangible closure,

a pusher piston sabot disposed between said propellant ant the rear end of said projectile,

said projectile being a flechette having a blunt tapered nose end, a cylindrical central shank section, and a stabilizing finned rear section, said finned rear section being cradled in a forward recess formed in said bore,

a further projectile-supporting unit disposed in and engaging said bore and engaging and supporting a forward portion of said projectile,

said further projectile-supporting unit being formed of soft elastic material and disposed about a portion of the nose end of said projectile and between the nose end of said projectile and the forward closed end of said barrel, and

said elastic projectile-supporting unit having a central thinwalled central membrane against which is disposed in axial compressive cradled relation the forward blunt nosed end of said projectile, the forward surface of said elastic projectile-supporting unit having a central relief configuration.

2. Underwater ammunition according to claim I,

said central relief configuration formed in the forward surface of said elastic projectile-supporting unit being an outwardly flared recess formed in spaced relation from the rear projectile-noseengaging surface of said unit by a thin-walled elastic membrane of smaller diameter than the blunt end face of said blunt tapered nose of said projectile.

3. Underwater ammunition according to claim 2, and

a central connecting bore formed in said elastic projectilesupporting unit between the forward wall of said elastic membrane and the inner mouth of said outwardly flared recess.

4. Underwater ammunition according to claim 2, and

a retainer disc disposed between said elastic projectile-supporting unit and the forward end of said barrel.

5. Underwater ammunition according to claim 4,

said retainer disc being a washer formed of relatively rigid frangible material and having a central aperture therein.

6. Underwater ammunition according to claim 5,

said frangible closure for said barrel being formed as an outwardly convex dome secured in watertight sealed relation at the forward end of said barrel.

7. Underwater ammunition according to claim 6,

said frangible convex dome closure being secured in watertight sealed relation in an end face recess formed in the forward end of said barrel.

8. Underwater ammunition according to claim 7,

said convex dome closure being spaced from said frangible retainer disc washer,

said barrel having a reduced diameter shoulder section disposed between said retainer disc washer and said frangible convex dome closure.

9. Underwater ammunition according to claim 2,

said pusher piston sabot having a plurality of helical shear threads formed on its outer annular surface for impact stopping of said sabot and for sealing propellant gases upon impact shearing, and a length of malleable material disposed in a portion of said threads for aiding in propellant gas sealing upon sabot impact, and retainer means impact stopping of said sabot by contact therewith of said threads.

10. Underwater ammunition according to claim 1, and

a further projectile-guiding-and-supporting unit disposed rearwardly of said projectile-supporting unit and including a thin frangible metal disc disposed in contiguous interfacing relation with and between two cylindrical sections of foam plastic,

said foam plastic sections being removably secured about said projectile, and,

said frangible metal disc having a central aperture through which said projectile extends in a light slip fit.

1 1. Underwater ammunition according to claim 1, and

a shear pin connection formed between said pusher piston sabot and said barrel.

12. Underwater ammunition comprising:

a smooth bore disposable barrel having a longitudinal bore closed at both ends and having a primer and propellant disposed therein at one end and a frangible closure at its opposite end,

a blunt tapered nosed projectile disposed in an air chamber formed in said bore between said propellant and said frangible closure, and

a pusher piston sabot disposed between said propellant and the rear end of said projectile,

said projectile being a flechette having a blunt tapered nose end, a cylindrical central shank section, and a stabilizing finned rear section, said finned rear section being cradled in a forward recess formed in said bore, and

a further projectile-supporting unit disposed in and engaging said bore and engaging and supporting a forward portion of said projectile, and

a shear pin connection formed between said pusher piston sabot and a separable rear section of said barrel.

13. Underwater ammunition according to claim 12,

the rear end of said projectile,

said pusher piston sabot having a plurality of helical shear threads formed on its outer annular surface for impact stopping of said sabot and for sealing propellant gases upon impact shearing, and a length of malleable material disposed in a portion of said threads for aiding in propellant gas sealing upon sabot impact, and retainer means impact stopping of said sabot by contact therewith of said threads. 

1. Underwater ammunition comprising: a disposable barrel having a longitudinal bore closed at both ends and having a primer and propellant disposed therein at one end and a frangible closure at its opposite end, a blunt tapered nosed projectile disposed in an air chamber formed in said bore between said propellant and said frangible closure, a pusher piston sabot disposed between said propellant ant the rear end of said projectile, said projectile being a flechette having a blunt tapered nose end, a cylindrical central shank section, and a stabilizing finned rear section, said finned rear section being cradled in a forward recess formed in said bore, a further projectile-supporting unit disposed in and engaging said bore and engaging and supporting a forward portion of said projectile, said further projectile-supporting unit being formed of soft elastic material and disposed about a portion of the nose end of said projectile and between the nose end of said projectile and the forward closed end of said barrel, and said elastic projectile-supporting unit having a central thinwalled central membrane against which is disposed in axial coMpressive cradled relation the forward blunt nosed end of said projectile, the forward surface of said elastic projectile-supporting unit having a central relief configuration.
 2. Underwater ammunition according to claim 1, said central relief configuration formed in the forward surface of said elastic projectile-supporting unit being an outwardly flared recess formed in spaced relation from the rear projectile-nose-engaging surface of said unit by a thin-walled elastic membrane of smaller diameter than the blunt end face of said blunt tapered nose of said projectile.
 3. Underwater ammunition according to claim 2, and a central connecting bore formed in said elastic projectile-supporting unit between the forward wall of said elastic membrane and the inner mouth of said outwardly flared recess.
 4. Underwater ammunition according to claim 2, and a retainer disc disposed between said elastic projectile-supporting unit and the forward end of said barrel.
 5. Underwater ammunition according to claim 4, said retainer disc being a washer formed of relatively rigid frangible material and having a central aperture therein.
 6. Underwater ammunition according to claim 5, said frangible closure for said barrel being formed as an outwardly convex dome secured in watertight sealed relation at the forward end of said barrel.
 7. Underwater ammunition according to claim 6, said frangible convex dome closure being secured in watertight sealed relation in an end face recess formed in the forward end of said barrel.
 8. Underwater ammunition according to claim 7, said convex dome closure being spaced from said frangible retainer disc washer, said barrel having a reduced diameter shoulder section disposed between said retainer disc washer and said frangible convex dome closure.
 9. Underwater ammunition according to claim 2, said pusher piston sabot having a plurality of helical shear threads formed on its outer annular surface for impact stopping of said sabot and for sealing propellant gases upon impact shearing, and a length of malleable material disposed in a portion of said threads for aiding in propellant gas sealing upon sabot impact, and retainer means impact stopping of said sabot by contact therewith of said threads.
 10. Underwater ammunition according to claim 1, and a further projectile-guiding-and-supporting unit disposed rearwardly of said projectile-supporting unit and including a thin frangible metal disc disposed in contiguous interfacing relation with and between two cylindrical sections of foam plastic, said foam plastic sections being removably secured about said projectile, and, said frangible metal disc having a central aperture through which said projectile extends in a light slip fit.
 11. Underwater ammunition according to claim 1, and a shear pin connection formed between said pusher piston sabot and said barrel.
 12. Underwater ammunition comprising: a smooth bore disposable barrel having a longitudinal bore closed at both ends and having a primer and propellant disposed therein at one end and a frangible closure at its opposite end, a blunt tapered nosed projectile disposed in an air chamber formed in said bore between said propellant and said frangible closure, and a pusher piston sabot disposed between said propellant and the rear end of said projectile, said projectile being a flechette having a blunt tapered nose end, a cylindrical central shank section, and a stabilizing finned rear section, said finned rear section being cradled in a forward recess formed in said bore, and a further projectile-supporting unit disposed in and engaging said bore and engaging and supporting a forward portion of said projectile, and a shear pin connection formed between said pusher piston sabot and a separable rear section of said barrel.
 13. Underwater ammunition according to claim 12, said shear pin connection comprising a shear pin extending thRough the rear section of said pusher piston sabot and into said separable rear section of said barrel, said separable rear section being the rear closure means for said barrel.
 14. Underwater ammunition comprising: a disposable barrel having a longitudinal smooth bore closed at both ends and having a primer and propellant disposed therein at one end and a frangible closure at its opposite end, a pusher piston sabot disposed between said propellant and the rear end of said projectile, said pusher piston sabot having a plurality of helical shear threads formed on its outer annular surface for impact stopping of said sabot and for sealing propellant gases upon impact shearing, and a length of malleable material disposed in a portion of said threads for aiding in propellant gas sealing upon sabot impact, and retainer means impact stopping of said sabot by contact therewith of said threads. 